The presence of 11-cis retinal, C20H28O, in cells in the eye is important for vision.
The structure of retinal includes an aldehyde group, a cyclohexene ring and a long aliphatic side
chain, in which a carbon-carbon double bond exists between carbons numbered 11 and 12.
Which pair of statements about 11-cis retinal could be correct?
5 C=C bonds and R1/R2 around the C=C bond between carbons 11 and 12 being on the same side
5 C=C bonds and R1/R2 around the C=C bond between carbons 11 and 12 being on opposite sides side
6 C=C bonds and R1/R2 around the C=C bond between carbons 11 and 12 being on the same side
6 C=C bonds and R1/R2 around the C=C bond between carbons 11 and 12 being on opposite side
I automatically knew that the R1/R2 hydrocarbon groups had to be on the same side because it's a cis compound, which eliminates two of the answers, but then how do you figure out the number of C=C double bonds with only that information and the molecular formula?
(1)For an alkane which only has a straight main chain with 20 carbon atoms, there will be 20*2 +2 = 42 hydrogen atoms. (2) When adding a ring into the main chain, subtract 2 hydrogen atoms. This is because when adding the ring, you are actually adding two alkyl groups; consequently, the two previous hydrogen atoms boning to the carbon on which you add a ring are actually replaced by the two 'alkyl groups'. (3) When having an aldehyde, two hydrogen atoms are removed, with one lost due to the forming of a double bond between a carbon atom and an oxygen atom, and with one lost owing to the oxygen atom. (4) When adding an alkene functional group, two hydrogen atoms are lost, as can be demonstrated by the fact that ethane has six hydrogen atoms in total but ethene only has four hydrogen atoms. (5) Hence, according to the aforementioned four reasons, we can firstly subtract 2 H from the 42 hydrogen atoms due to the presence of an aldehyde group; secondly, we subtract 2 H form the remaining 40 hydrogen atoms, as a cyclohexene ring presents; and then we know that there are 38-28=10 hydrogen atoms lost because of the presence of alkene groups. Therefore, divide 10 by 2; the result is 5, meaning that there are five double bonds (c=c).
For 20carbon means that there are 40Hyrdrogen.
Number of hydrogen left =40-28=12
To make an aldehyde, 2H is used
Number of hydrogen left after aldehye is made = 12-2 = 10
To make alkene (double bond), 2H is used for one double bond
Since there are 10hydrogen left,
10/2=5
There are 5 C=C and C=O
Why divide by 2. And why are 40 carbons there r 28 of them and if there are 40 why subtracted 28!!!!!!!
To determine the number of carbon-carbon double bonds (C=C) in 11-cis retinal, you can use the molecular formula (C20H28O) and the knowledge of its structural features.
First, determine the number of hydrogen (H) atoms present in the molecule. Since every carbon atom is bonded to either a hydrogen or another carbon, the number of hydrogen atoms can be calculated using the formula:
Number of Hydrogen atoms (H) = 2 x Number of Carbon atoms (C) + 2
In this case,
Number of Hydrogen atoms = 2 x 20 + 2 = 42
Now, calculate the total number of hydrogen atoms in the molecule (Htotal) by subtracting the number of hydrogen atoms from the molecular formula:
Htotal = 28 - 42 = -14
This negative value suggests that there is an incorrect molecular formula or a mistake in the information provided. As a result, we cannot accurately determine the number of carbon-carbon double bonds based solely on the given information.
However, assuming there was a typo in the molecular formula and it is supposed to be C20H28O, we can proceed with the calculation.
To find the number of carbon-carbon double bonds, you can use the information about the aliphatic side chain. Since the aliphatic side chain contains the double bond between carbons numbered 11 and 12, this tells us that there must be at least one carbon-carbon double bond in the molecule.
Based on the remaining answer choices, you have two options left: 5 C=C bonds or 6 C=C bonds.
To differentiate between these two options, you can consider the overall structure of retinal. The presence of the cyclohexene ring suggests the existence of a carbon-carbon double bond within it. Since the aliphatic side chain has one double bond, the remaining double bonds must be present within the cyclohexene ring.
Knowing that a cyclohexene ring contains three double bonds, this means that there are in total six carbon-carbon double bonds in 11-cis retinal.
Therefore, the correct pair of statements about 11-cis retinal would be:
6 C=C bonds and R1/R2 around the C=C bond between carbons 11 and 12 being on the same side.